The problem of controlling static charge is well known in the field of photography. The accumulation of charge on film or paper surfaces leads to the attraction of dirt which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. Static problems have been aggravated by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic film product by both the customer and photofinisher. In an automatic camera, the winding of roll film in and out of the film cartridge, especially in a low relative humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films (e.g., x-ray films) are especially susceptible to static charging during removal from light-tight packaging.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive "antistatic" layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outermost coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors.
Most of the traditional antistatic layers comprise ionic conductors. Thus, charge is transferred in ionic conductors by the bulk diffusion of charged species through an electrolyte. The prior art describes numerous simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols stabilized by metal salts. Conductivity of most ionically conductive antistatic agents is generally strongly dependent upon temperature and relative humidity of the environment as well as the moisture in the antistatic layer. Because of their water solubility, many simple ionic conductors are usually leached out of antistatic layers during processing, thereby lessening their effectiveness.
Antistatic layers employing electronic conductors have also been described in the art. Because the conductivity depends predominantly upon electronic mobilities rather than ionic mobilities, the observed electronic conductivity is independent of relative humidity and other environmental conditions. Such antistatic layers can contain high volume percentages of electronically conductive materials including metal oxides, doped metal oxides, conductive carbon particles or semi-conductive inorganic particles. While such materials are less affected by the environment, a lengthy milling process is often required to reduce the particle size range of oxides to a level that will provide a transparent antistatic coating needed in most imaging elements. Additionally, the resulting coatings are abrasive to finishing equipment given the high volume percentages of the electronically conductive materials.
Electrically-conductive polymers have recently received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for photographic applications, some of these electrically-conductive polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 4,731,408; 4,959,430; 4,987,042; 5,035,926; 5,300,575; 5,312,681; 5,354,613; 5,370,981; 5,372,924; 5,391,472; 5,403,467; 5,443,944; 5,463,056; 5,575,898; and 5,747,412) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550 and 5,093,439) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conductive even at low humidity. Moreover, these polymers can retain sufficient conductivity even after wet chemical processing to provide what is known in the art as "process-surviving" antistatic characteristics to the photographic support they are applied onto. Unlike metal-containing semiconductive particulate antistatic materials (e.g., antimony-doped tin oxide), the aforementioned electrically-conductive polymers are less abrasive, environmentally more acceptable (due to the absence of heavy metals), and, in general, less expensive.
However, it has been reported that the mechanical strength of a binderless antistat layer comprising substituted or unsubstituted thiophene-containing polymers is not sufficient and can be easily damaged unless a water-soluble or water-dispersible binder is used in the antistat layer (U.S. Pat. Nos. 5,300,575 and 5,354,613). Alternatively, the mechanical strength of an antistat layer comprising only substituted or unsubstituted thiophene-containing polymers can be improved by applying an overcoat layer of a film-forming polymeric material from either an organic solvent solution or an aqueous solution or dispersion (U.S. Pat. No. 5,370,981). A preferred polymeric material for use as an aqueous dispersible binder with such polythiophene containing antistatic layers, or as a protective overcoat layer on such polythiophene-containing antistatic layers is polymethyl methacrylate (U.S. Pat. Nos. 5,354,613 and 5,370,981). However, these binders or protective overcoat layers may be too brittle for certain applications, such as motion picture print films (as illustrated in U.S. Pat. No. 5,679,505).
Alternative polymeric materials for overcoats include cellulose derivatives, polyacrylates, polyurethanes, lacquer systems, polystyrene or copolymers of these materials (as discussed in U.S. Pat. No. 5,370,981). However, according to U.S. Pat. No. 5,370,981, the use of an alkoxysilane is required in either the binderless polythiophene containing antistatic layer, the overcoat layer, or both layers to provide layer adhesion in such a two layer structure.
A variety of water-soluble or water-dispersible polymeric binder materials have been used in polythiophene containing antistat layers. In addition to the aforementioned polymethylmethacrylate, water dispersible materials include hydrophobic polymers with a glass transition temperature (Tg) of at least 40.degree. C. such as homopolymers or copolymers of styrene, vinylidene chloride, vinyl chloride, alkyl acrylates, alkyl methacylates, polyesters, urethane acrylates, acrylamide, and polyethers (as discussed in U.S. Pat. No. 5,354,613). Other water dispersible materials include polyvinylacetate (U.S. Pat. No. 5,300,575) or latex (co)polymers having hydrophilic functionality from groups such as sulphonic or carboxylic acid (U.S. Pat. No. 5,391,472). Water soluble binders include gelatin and polyvinylalcohol (U.S. Pat. No. 5,312,681). Polythiophene containing antistat layers, both in the presence and absence of water-soluble or water-dispersible polymeric binder materials, have been shown to tolerate the addition of water-miscible organic solvents (U.S. Pat. No. 5,300,575). However, the prior polythiophene antistat art only teaches the use of polythiophene in combination with water-soluble or water-dispersible polymeric binder materials prepared via solutions containing a minimum water content of approximately 37 wt % (as seen in U.S. Pat. No. 5,443,944, column 7, lines 1-17, magnetic and antistat layer 6.3 in Example 6).
Prior art for substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654) describes the use of these materials dispersed in a film-forming binder. While a broad range of binders useful in antistatic layers is described, examples from these patents only teach the use of aqueous coatings containing polypyrrole and water-dispersible or water-soluble binders.
Prior art for substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. No. 5,716,550) describes the use of these materials dissolved in a first solvent and a film-forming binder dissolved in a second different solvent. While the above art teaches the use of cellulosic film-forming binders with substituted or unsubstituted aniline-containing polymers, in order to prepare the antistat layer this art teaches the use of solvent systems such as chlorinated solvents, which are environmentally less friendly. In addition, examples from this art indicate a light green color even at coverages of the substituted or unsubstituted aniline-containing polymer as low as 0.01 g/m.sup.2.
What is needed in the art is an imaging element comprised of an electrically-conductive antistatic layer that confers on the element process-surviving antistatic characteristics as well as resistance to abrasion and scratching and improved layer adhesion, without adding undesirable coloration to the imaging element.